The invention relates to a method of minimizing errors in a rotor angle estimate in a synchronous machine, the method comprising the steps of
determining the stator flux of the synchronous machine on the basis of the current model of the synchronous machine,
determining the stator flux of the synchronous machine on the basis of the voltage integral and the initial value of the stator flux, and
determining the rotor angle of the synchronous machine.
A synchronous machine generally consists of a stator provided with three-phase winding and a magnetized rotor. The rotor is typically magnetized either by means of permanent excitation or separate excitation. In permanent excitation the rotor is provided with permanent magnet blocks, which the magnetic field produced in the stator pulls towards itself, simultaneously rotating the rotor. Separate excitation of the rotor means that inside the rotor there are coils of wire to which power is applied. Thus the coils of wire in the rotor form magnetic poles in the rotor, which function according to the same principle as poles made of permanent magnets. The rotor of the synchronous machine may be a salient-pole rotor or a cylindrical rotor. In cylindrical rotor machines the rotor inductance is almost constant with respect to the stator, whereas in salient-pole machines the rotor inductance varies a lot because of changes in the air slot between the rotor and the stator, depending on the rotor position angle.
In speed-controlled synchronous machines it is important in respect of the function of the control system that the position angle of the machine""s rotor is known as accurately as possible. Particularly in control methods based on direct control of the machine""s stator flux the accuracy of angle determination has a considerable. influence on the accuracy of control. Usually a pulse encoder or an absolute sensor is used for determining the rotor position angle, and the information obtained from them can be used for determining the rotor angle.
The measurement result obtained from the angle sensor contains errors caused by at least two components that can be determined. The first one of the known components causing errors is incorrect initial angle, which is determined by an angle sensor. Various estimation algorithms have been presented for estimating the initial angle. However, the rotor can be initially turned in the desired direction, provided that the motor load allows this. The rotor is preferably turned so that it is in the same direction as the coil of a phase, for example. The rotor can be turned by supplying direct current to the desired phase, and thus the rotor turns in the desired direction. However, determination and correction of the initial angle by turning the rotor is often impossible because of the applications for which synchronous machines are used.
The error in the rotor angle may also result from delays in the discrete control system. If the rotor angle used for calculating the motor state has been determined in relation to time xcex94T before calculating the motor model, this causes an additional error which is proportional to the angle frequency of the rotor according to the equation xcex94xcex8=xcfx89xcex94t. In other words, the angle information determined as the rotor rotates becomes outdated before it is used for calculating the motor model. Thus the angle error caused by the delays is directly proportional to the angle speed of the rotor.
In motor control methods based on direct control of the torque caused by the stator current of the synchronous machine, the voltage to be applied to the stator is selected on the basis of a flux estimate "psgr"ssu which is based on the voltage integral and calculated on the basis of a short sampling period. The flux estimate "psgr"ssu is determined using integral theorem (1)       Ψ    su    s    =            Ψ      su0      s        +                  ∫        t                  t          +                      Δ            ⁢                          xe2x80x83                        ⁢            t                              ⁢                        (                                    u              s              s                        -                                          R                s                            ⁢                              i                s                s                                              )                ⁢                  ⅆ          t                    
where "psgr"ssu0 is the initial value of the flux, uss is the voltage to be applied, iss is the determined stator current and Rs the stator resistance. In the complex plane plane the flux estimate "psgr"ssu is described by an oscillating circular path because the flux is controlled within certain hysteresis limits. If the flux estimate "psgr"ssu is equal to the real flux "psgr"ss in the stator, the track of the phasor iss of the stator current to be generated is an origin-centred circle. Because of an erroneous voltage or resistance estimate the real flux "psgr"ss may, however, follow a track which differs from that of the estimate.
An estimate "psgr"ssest based on the current model of the motor is used to stabilize the estimate based on the integral (2)
xe2x80x83"psgr"ssest=ƒ[L1(xcex8r), L2(xcex8r), . . . , Ln(xcex8r), is, "psgr"71 (xcex8r)]
where L1(xcex8r), L2(xcex8r), . . . , Ln(xcex8r)represent the inductances that affect the stator flux, xcex8r is the rotor angle and "psgr"ƒ(xcex8r) is the flux, such as separate or permanent excitation, which is caused by the rotor and is independent of the stator current. The flux estimates produced by the models are compared with each other and the difference is updated in the estimate produced by the voltage model. As it appears from formula (2), in the synchronous machine the current model requires information of the rotor angle expressed in stator coordinates. If the angle information received from the pulse encoder or the absolute sensor is erroneous, then the flux estimate will also be erroneous, and consequently the update made in the voltage model is erroneous. Even though the above-mentioned manner provides a reliable estimate of the magnitude of the stator flux, the accuracy of the torque and speed control of operation is significantly impaired if the angle error in the rotor is considerable.
The object of the present invention is to provide a method which allows to avoid the above-mentioned drawbacks and to estimate the rotor angle of a synchronous machine accurately, thus enabling reliable and accurate control of the operation of a synchronous machine. This object is achieved with a method according to the invention, which is characterized in that the method comprises the step of
determining the correction term of the angle on the basis of the determined stator fluxes, and
adding the correction term of the angle to the determined rotor angle of the synchronous machine to obtain a rotor angle estimate.
The method of the invention is based on the idea that an exact estimate is obtained for the rotor angle when a correction term xcex8r is added to a determined rotor angle, the correction term being proportional to the difference of the direct-axis components of the stator fluxes determined on the basis of the current and voltage models of the synchronous machine.
The method of the invention provides a reliable estimate of the real rotor position angle of the synchronous machine by means of simple operations, which is important to obtaining accurate controllability of the synchronous machine. Thanks to its simplicity, the method is easy to implement and does not require additional measurement or parameter information.